1. Field of the Invention
This invention relates to an optical recording medium having a recording layer to be recorded by heat mode recording such as a phase change recording layer or a magnetooptical recording layer, and a method for producing such medium.
Optical information media such as read-only optical disks and optical recording disks have been required to have a higher capacity by increasing the recording density for the purpose of recording and storing an enormous amount of information as in the case of motion picture information. Extensive efforts have been dedicated to the research and development of the recording at a higher density to meet such request.
2. Prior Art
Under such situation, one proposal has been use of a smaller laser beam spot with a reduced diameter in the recording and reading as in the case of DVD (Digital Versatile Disk) by reducing the wavelength used in the recording/reading and increasing the numerical aperture (NA) of the objective lens of the recording/reading optical system. When the DVD is compared to CD, the DVD has realized a recording capacity (of 4.7 GB/side) which is 6 to 8 times larger than that of the CD by reducing the recording/reading wavelength from 780 nm to 650 nm and by increasing the NA from 0.45 to 0.6.
Use of a higher NA, however, invites decrease of tilt margin. Tilt margin is tolerance for the tilting of the optical information medium in relation to the optical system, and the tilt margin is determined by the NA. When the recording/reading wavelength is xcex, and the transparent substrate through which the medium is irradiated with the recording/reading beam has a thickness t, the tilt margin is proportional to
xcex/(txc2x7NA3)
Tilting of the optical recording medium at an angle to the laser beam, namely, occurrence of the tilt results in the generation of wave front aberration (coma aberration). When the substrate has a refractive index of n and a tilt angle of xcex8, the wave front aberration coefficient is given by
(xc2xd)xc2x7txc2x7{n2xc2x7sin xcex8xc2x7cos xcex8}xc2x7NA3/(n2xe2x88x92sin2 xcex8)xe2x88x925/2
These relations indicate that decrease in the thickness t of the substrate is effective when the tilt margin is to be increased with simultaneous suppression the generation of the coma aberration. As a matter of fact, tilt margin is ensured in the case of DVD by reducing the thickness of the substrate to about half (about 0.6 mm) of the thickness of the CD (about 1.2 mm). In the meanwhile, margin of the thickness unevenness of the substrate is given by
xcex/NA4
When the substrate has an uneven thickness, such uneven thickness further results in wave front aberration (spherical aberration). When the substrate has a thickness unevenness of xcex94t, the spherical aberration coefficient is given by
{(n2xe2x88x921)/8n3}xc2x7NA4xc2x7xcex94t
As indicated by these relations, the thickness unevenness of the substrate should be reduced in order to reduce the spherical aberration associated with the increase in the NA. For example, in the case of DVD, xcex94t is suppressed to xc2x130 xcexcm compared to that of xc2x1100 xcexcm in the CD.
A structure enabling further decrease in the substrate thickness has been proposed in order to realize high quality motion picture recording for a longer period. In this structure, a substrate having normal thickness is used as a supporting substrate for ensuring rigidity of the medium, and the pits and the recording layer are formed on its surface, and a light-transmitting layer in the form of a thin substrate having a thickness of about 0.1 mm is formed on the recording layer. The medium is irradiated with the recording/reading beam through this light-transmitting layer. This structure enables drastic reduction in the thickness of the substrate, and high density recording by the use of a higher NA is thereby enabled. A medium having such structure is described, for example, in Japanese Patent Application Laid-Open Nos. (JP-A) 320859/1998 and 120613/1999.
The medium described in JP-A 320859/1998 is a magnetooptical recording medium, and this magnetooptical recording medium has a structure wherein a metal reflective layer, a first dielectric layer, a magnetooptical recording layer, a second dielectric layer, and a light-transmitting layer are disposed on the substrate in this order. In JP-A 320859/1998, surface roughness of the metal reflective layer at the interface between the dielectric layer and the metal reflective layer is reduced to the level of less than 8.0 nm based on the view that increase in the signal noise in the reading is induced by the excessively large surface roughness of the metal reflective layer formed by sputtering. In JP-A 320859/1998, an aluminum-containing material, and preferably, a material containing aluminum in admixture with at least one member selected from Fe, Cr, Ti and Si, or Au or Ag is used for constituting the metal reflective layer, and ion beam sputtering or magnetron sputtering is employed for the layer formation.
The medium described in JP-A 120613/1999 is a phase change optical recording medium wherein the medium is formed by disposing a reflective layer, a phase change recording layer, and a light-transmitting layer on the substrate in this order. This medium also reduces the surface irregularity of the reflective layer by adopting the reflective layer of particular composition. There is stated in JP-A 120613/1999 that xe2x80x9cmorphology of the boundary reflecting the grain size determined by the crystallinity of the reflective layer and the composition of the reflective layerxe2x80x9d, and therefore, it is understood that the surface roughness of the metal reflective layer is reduced in JP-A 120613/1999 by reducing the grain size.
In a phase change optical recording medium or a magnetooptical recording medium, the dielectric layer and the as deposited recording layer are amorphous, and their surface are very smooth by nature. In contrast, in the medium described in the JP-A 320859/1998 and JP-A 120613/1999, the dielectric layer and the recording layer are formed on the reflective layer, and the surface roughness of the reflective layer will be transferred to the dielectric layer and the recording layer. Accordingly, when the reflective layer has a surface roughness of considerable level, the overlying layers will also have the surface roughness of corresponding level and the boundary between the layers will be irregular. The laser beam will then be reflected at various layers, and the laser beam reflected from the medium which has undergone interference will include scattered light generated as a result of surface roughness at the boundary between the layers. It is believed that the noise in the reading is thereby increased. In view of such situation, reduction of the surface roughness of the reflective layer as described in JP-A 320859/1998 and JP-A 120613/1999 are quite effective.
However, in the investigation of the inventors of the present invention, it has been found out that the phase change optical recording medium and the magnetooptical recording medium provided with the reflective layer having an excessively small grain size suffer from insufficient recording characteristics due to insufficient heat conductivity of the reflective layer. There is a demand for further reduction in the surface roughness of the reflective layer in consideration of the ongoing shortening of the wavelength of the recording/reading laser, and in such case, heat conductivity of the reflective layer will be further reduced.
In view of the situation as described above, an object of the present invention is to provide an optical recording medium which has a light-transmitting substrate, a recording layer recorded by heat mode recording, and a reflective layer comprising a metal or a semimetal disposed in this order, and which exhibits reduced signal noise in the reading as well as improved recording characteristics.
Such objects are attained by the present invention as described in (1) to (13), below.
(1) A method for producing an optical recording medium having at least a light-transmitting substrate, a recording layer recorded by heat mode recording, and a reflective layer comprising a metal or a semimetal disposed in this order, wherein the method includes the step of
changing bonding state of the element(s) constituting the reflective layer at least in the area to be recorded.
(2) The method for producing an optical recording medium according to the above (1) wherein the optical recording medium has a supporting substrate, and the medium is the one produced by forming the reflective layer, the recording layer, and the light-transmitting substrate on the supporting substrate in this order.
(3) The method for producing an optical recording medium according to the above (1) wherein, in said step of changing the bonding state of the element(s) constituting the reflective layer, the reflective layer is changed from amorphous phase to crystalline phase at least in the area to be recorded.
(4) The method for producing an optical recording medium according to the above (1) wherein, in said step of changing the bonding state of the element(s) constituting the reflective layer, grain size of the reflective layer is increased at least in the area to be recorded.
(5) The method for producing an optical recording medium according to the above (1) wherein, in said step of changing the bonding state of the element(s) constituting the reflective layer, heat conductivity of the reflective layer is increased at least in the area to be recorded.
(6) The method for producing an optical recording medium according to the above (1) wherein the reflective layer contains at least two elements.
(7) The method for producing an optical recording medium according to the above (1) wherein said recording layer is a phase change recording layer, and the step of heat treatment carried out for the purpose of changing the amorphous recording layer into crystalline state is used as said step of changing the bonding state of the element(s) constituting the reflective layer.
(8) An optical recording medium produced by the method of the above (1).
(9) An optical recording medium having at least a light-transmitting substrate, a recording layer recorded by heat mode recording, and a reflective layer comprising a metal or a semimetal disposed in this order, wherein
bonding state of the element(s) constituting the reflective layer is different at least between the area to be recorded and other area, and
the medium is used such that the reading beam enters the medium through the light-transmitting substrate.
(10) The optical recording medium according to the above (9) wherein the reflective layer is crystalline at least in the area to be recorded, and amorphous in other area.
(11) The optical recording medium according to the above (9) wherein grain size of the reflective layer is greater at least in the area to be recorded compared to the grain size of the reflective layer in other area.
(12) The optical recording medium according to the above (9) wherein heat conductivity of the reflective layer is greater at least in the area to be recorded compared to the heat conductivity of the reflective layer in other area.
(13) The optical recording medium according to the above (9) wherein the reflective layer comprises at least two elements.